1. Field of the Invention
The instant invention relates to a process for the purification of dimethylether, which contains impurities, by feeding a mixture which contains the dimethylether at specific trays to a distillation column and withdrawal of the dimethylether and of impurities at specific trays of the same column.
2. Description of the Prior Art
Before the development of low pressure processes for the production of methanol, dimethylether was obtained as a side-product of high pressure methanol processes, usually in a quantity of 2 to 3 weight-% based on the total quantity of products withdrawn from the methanol synthesis reactor. Dimethylether was separated from raw methanol together with other impurities as first runnings in the distillation unit for the production of pure methanol.
After introduction of low pressure methanol processes, which form only negligible quantities of dimethylether, specific processes for the synthesis of dimethylether by catalytic dehydration of methanol were developed.
Numerous catalysts and processes are disclosed in the patent literature.
According to DE-PS No. 680 328 aliphatic ethers are obtained by heating alcohols in the presence of ZnCl.sub.2. Other suitable catalysts for the production of ethers from alcohols are according to British Patent Publications No. 332 756, No. 350 010 and No. 403 402, U.S. Pat. No. 1,873,537 and French Patent Publication No. 701 335 ferric chloride, copper sulfate, stannic chloride, manganese chloride, aluminum chloride and sulfate, chromium sulfate, alums, thorium derivates, aluminum oxide, titanium oxide, barium oxide, silica gel or aluminum phosphate.
The synthesis of dimethylether directly from synthesis gas (CO+H.sub.2) has also been described (DE-PS No. 23 62 944, DE-PS No. 27 57 788 and DE-PS No. 32 20 547).
The technically most important catalysts have proven to be according to DE-PS No. 28 18 831, DE-OS No. 32 01 155, European Patent Application No. 0 099 676 and European Patent Application No. 0 124 078 in particular aluminum oxide and aluminum silicate catalysts with and without doping.
The raw dimethylether thus obtained contains reaction water, unconverted methanol as well as small quantities of impurities like for example methylformate, hydrocarbons, amines and sulfides.
In these production plants raw dimethylether is worked up in two consecutive distillation columns, in the first of which, dimethylether is distilled off under pressure and in the second of which unconverted methanol is separated and recycled.
Thus in European Patent Application No. 0 124 078 a process is disclosed, according to which in a first pressurized column, dimethylether is separated as a side-stream, whereas in a second column, which is operated at a lower pressure, the impurities with boiling points between methanol and dimethylether are withdrawn as a head product. Unconverted methanol is separated also in the second column as a side-product.
Although this process leads to dimethylether of high purity, it has the considerable economic disadvantage, that not only the first but also the second column have to be provided with a high number of trays, resulting in high capital expenditure and in particular in high operating costs. Furthermore there is a risk that impurities with boiling points between dimethylether and methanol remain at least in small quantities in the first column resulting in contamination of dimethylether.
Since dimethylether is of increasing importance as a propellant in aerosol sprays, very high demands are made on the purity of dimethylether for certain applications. In particular for cosmetic, human and household applications dimethylether must be free of irritating substances. Furthermore dimethylether must be odorless for these applications. Thus the problem existed, to make available on the one hand a more economical production process for dimethylether compared to the state of the art and on the other hand to produce dimethylether in high purity and practically quantitatively based on feed methanol.